This invention relates generally to a ballast for a fluorescent lamp, and more particularly to a triac dimmable, single stage compact fluorescent lamp.
A conventional single stage compact fluorescent lamp, such as disclosed in PCT Patent Application No. WO 96/07297, is designed to provide a high input power factor (p.f) ranging from about 0.93 to 0.99 with low total harmonic distortion (THD). The lamp ballast includes only an output (i.e. inverter) stage commonly referred to as a single stage compact fluorescent lamp. There is no need for an additional stage such as a switch mode power supply for maintaining high p.f Instead, the lamp ballast includes a feedback capacitor for drawing sufficient current from the ballast input to maintain high p.f. A resonant tank circuit is designed to operate in an inductive mode so as to maintain zero voltage switching (ZVS). A voltage across a buffer capacitor of the lamp ballast is generally above its nominal rating during preheat and ignition of the lamp. Consequently, the ballast components must be designed to withstand the voltage stress experienced during lamp start-up.
Unfortunately, the feedback capacitor is undesirably large in size for a CFL in order to draw sufficient current to maintain high p.f. The resonant tank circuit, which serves to maintain ZVS, must be oversized and is relatively expensive. The lamp ballast also includes a choke to compensate for the capacitive impedance of the inverter output. Otherwise, the size of the resonant tank circuit would need to be further increased to achieve ZVS. The need for the choke also undesirably further increases the CFL space requirements.
The lamp ballast is not designed for triac dimming. In particular, unacceptably high overboost voltages will occur across the buffer capacitor during low level dimming (e.g. at about 10% of full light output). At such low dim levels, it can be difficult to continue to draw current through the triac at or above the triac holding current to maintain triac conduction.
The values of many of the components within a conventional triac dimmable CFL also must be changed based on the type of triac and lamp (i.e. burner) employed. The need to change the values of these components in order to accommodate different types of triacs and burners makes it difficult to modularize the feedback components.
Accordingly, it is desirable to provide an improved triac dimmable CFL in which both the buffer capacitor and resonant tank circuit can be significantly reduced in size. Overboost voltages across the buffer capacitor should be minimized especially during low dim levels while continuing to draw current through the triac at or above the triac holding current to maintain triac conduction. The improved triac dimmable CFL, in particular, should provide modularity in regard to the feedback components.